The invention relates to a method of measuring optical radiation, in which method the intensity of a radiation coming from an object to be measured and to be lighted by collimated radiation is measured at several wavelengths by focusing the radiation by an optical means and a mirror means on a detector group comprising several detector elements.
Spectroscopic measurements are based on an investigation of a spectrum of radiation coming from an object to be measured. It depends on the properties of the object to be measured how the spectral distribution of the radiation, which is reflected or emitted from the object or which has penetrated the object, is within a frequency range. For instance, the thickness of a film of slushing oil or impurities on the surface of the object influences the distribution and amplitude of the spectrum reflected from the object or, in practice, from the film on the surface.
Typical of spectroscopic measurements is that the radiation obtained from a spot-like light source is collimated, i.e. made parallel, and the object to be measured is lighted by this radiation.
Detectors used in spectroscopic measurements comprise several detector elements connected to each other, each detector element measuring radiation coming from an object within different wavelength ranges. For a successful spectroscopic measurement, each detector element should see the object at the same incidence angle, in order that the multichannel detector formed by the detector elements may be used for measuring relative intensities of different wavelengths of the spectrum of the radiation reflected. In practice, it is, however, necessary to position the detector elements relatively far from each other for structural technical reasons, due to which the detector elements see the same object at slightly different angles or the radiation comes to the separate detector elements at the same angle, but has started from another part of the object than the radiation arrived at the rest of the detector elements. These problems appearing as error factors in measurements cannot be eliminated by fixed optics.
To eliminate these problems, it is known to use a diffuser in front of the detector elements. A parallel radiation bundle coming to the diffuser is diffused in the diffuser, which leads to that the radiation coming to the elements is mixed in different directions. On account of the operating principle of the diffuser, a large part of the radiation, i.e. of the light, passes the detectors, and the diffuser attenuates also the intensity of the radiation, because the more light rays come to the diffuser, the more the radiation is attenuated.
U.S. Pat. No. 4,792,684 discloses a horizontal scanner, which is used for instance in satellites or missiles to follow their movements. The device described in this publication directs the light rays collected by the device from several different directions and objects to one detector element. By this solution, it is not possible to avoid dimensional errors, because the light rays come from different places, due to which a ratio measurement taking place within a certain wavelength range would give somewhat erroneous measurement results. The mirror means to be used in the solution according to this publication comprises two separate levels, which are positioned at an angle with respect to each other. By means of this solution, it is not possible to circulate a focus on a detector level formed by several detectors. In this solution, moreover, the reflecting mirror is tilted to form a big angle of about 30.degree. in relation to a level perpendicular to its rotating axis. The so-called split mirror structure causes a halving of the light intensity into two separate radiation beams.
U.S. Pat. No. 4,748,329 describes a method of and a system for measuring the thickness of a light transmitting film, whereby a multichannel detector measures radiation reflected from the surface. Several reflector means are used in this solution, by means of which the focus of radiation cannot be circulated by one mirror means on the same detector level over the separate detector elements.
German Patent 36 37 125 discloses a device for measuring reflection to be used in a spectrometer. In this device, reflection is measured at several different incidence angles and a corner prism included in the device returns the reflected light in the same direction, and then the light goes to a detector irrespective of a change in the incidence angle of the reflected radiation. By means of the simple device structure of the solution in question, it is not possible to provide a direction of radiation to a multi-channel detector in such a way that each detector element would see the radiation come to the detector elements at the same incidence angle with the same input aperture.
U.S. Pat. No. 4,923,263 discloses an optic scanner comprising two rotating mirrors tilted with respect to the rotating axis thereof, which scanner additionally comprises a field lens and a relaying transmission lens. The mirrors rotate at different speeds and at different phases, due to which several scanning figures of different shapes are obtained on the detector in the focal plane. This solution concerns a device with an operation similar to that of a camera, in which an image is produced on a first lens already and after that the image is transferred through mirrors to one and only detector. This solution does not show a focusing of a parallel radiation bundle as late as in the focal plane and an alternating circulation of the focus path created in this way over detector elements excited to different wavelengths, but a transfer of an image produced already earlier to the detector plane, i.e. to the focal plane. The dimensions of the image can be changed by means of a double-mirror structure.
U.S. Pat. No. 5,089,908 discloses an optic scanning system, by means of which a so-called plywood effect at image production shall be attenuated. The system comprises laser diodes at different wavelengths operating according to a control based on a video signal, the radiation of which diodes is directed through optics via a rotating polygon reflector over the detector plane. However, the structure of the rotating polygon reflector is in this solution such that it causes discontinuities in a light ray and losses in the radiation effect thereof, because the polygon reflector comprises numerous mirror surfaces, which, each in turn, reflect the radiation together with the rotating movement to the detector, whereby, when moving from one mirror surface to another, the radiation is interrupted and the focus of radiation returns to its starting point. This structure is suitable for image production for instance in copying machines, but not for spectroscopic applications. Also the basic structure of this system differs considerably from the present solution, for the system comprises several laser diodes operating at different wavelengths, and the system does not even comprise an object to be measured between the light source and the focusing optics and the rotating mirror.
Moreover, the publications WO 90/07697, U.S. Pat. Nos. 3,523,734, 4,687,329 and 5,050,991 disclose spectrometric methods and devices, in which radiation is deflected by means of a grid dispersing radiation, whereby the grid spreads the spot-like radiation to a line spectrum at the same time to the detectors, which is, however, no good solution in all applications.
Consequently, the solutions according to the prior art comprise a number of problems. The object of this invention is to set forth a novel method avoiding the problems associated with the known solutions.